1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method and a process cartridge.
2. Discussion of the Background
Recently, the development of the information processing systems using electrophotography has been notably significant. Among these, optical printers, which convert information into digital signals to optically record the information, have been extremely improved in terms of the quality of printing and reliability. This digital recording technology is applied to not only printers but also typical photocopiers, which leads to the development of digital photocopiers. In addition, it is anticipated that a typical analogue photocopier using this digital recording technology is more and more demanded because such a photocopier has various kinds of information processing functions. Further, with the diffusion and the improvement of performance of home computers, the development of a digital color printer for outputting color images and documents increasingly speeds up.
In an electrophotographic image bearing member for use in these systems and devices, in general, a function separation type layered image bearing member is pervasively used, in which a charge generating layer is provided on a substrate with an optional intermediate layer therebetween and a charge transport layer is provided on the charge generating layer. Furthermore, to improve the mechanical or chemical resistance of an image bearing member, a surface protective layer can be formed as the surface layer thereof. When a surface charged image bearing member of this type is irradiated with light, the light transmits a charge transport layer and reaches a charge generating material in the charge generating layer. The charge generating material absorbs the light and generates charge carriers. The generated charge carriers are infused into the charge transport layer and transferred therein along the electric field generated by the charging to neutralize the surface charges of the image bearing member. As a result, a latent electrostatic image is formed on the surface of the image bearing member. Therefore, a combination of a charge generating material which absorbs near infrared and optical light and a charge transport material which does not hinder the transmission of the light absorbed in the charge generating material, i.e., absorbs optical light (yellow light range) and ultraviolet, is used for the function separation type layered image bearing member in most cases.
As a light source dealing with such a digital recording system, for example, small, inexpensive and reliable semicondcutor lasers (LDs) and electroluminescence diodes (LEDs) are used in most cases. The oscillation wavelength of the most widely used LDs is in the near infrared range, i.e., from about 780 to about 800 nm. The luminous wavelength of a representative LED is 740 nm.
Recently, violet or blue LDs and LEDs having an oscillation wavelength shorter than 450 nm (i.e., from 375 to 450 nm) have been developed and introduced into the market as a writing light source dealing with the digital recording format for use in DVDs, etc. For example, when an LD having about a half oscillation wavelength in comparison with that of a typical near infrared LD is used as a writing light source, the spot diameter of the laser beam on an image bearing member can be theoretically significantly small as shown in the following relationship: d∝(π/4) (λf/D) (in the relationship, d represents the spot diameter of a laser beam on an image bearing member, λ represents the wavelength of a laser beam, f represents the focusing length of an fθ lens and D represents the diameter of a lens). This is extremely advantagenous to improve the writing density, i.e., definition, of a latent electrostatic image.
When a light source having a wavelength shorter than 450 nm is used as a writing source, it is possible to irradiate an image bearing member with light having a beam (dot) spot of about 30 μm corresponding to 1,200 dpi or about 15 μm corresponding to 2,400 dpi while maintaining the clearness of the contour thereof.
Improvement on the quality of images and the durability, including colorization, is demanded for the printer and the photocopier mentioned above. There are two issues for the improvement on the quality of images for a digital apparatus. One is how to uniformly form a latent electrostatic image with fine dots. The other is how to reduce the occurrence of various kinds of abnormal images. The quality of images can be improved by using the light source mentioned above emitting light having a short wavelength. However, the occurrence of various kinds of abnormal images has been remained unsolved.
With regard to the improvement on durability, it is highly effective to elongate the life of an image bearing member.
There are various kinds of approaches to solve these issues. The thing common in both issues is how to restrain the deterioration of an image bearing member caused by electrostatic fatigue for use in these image forming apparatuses. To be specific, it is how to restrain the rise of the voltage at irradiated portions during repetitive use.
To restrain the rise of the voltage at irradiated portions, the design (composition, structure, etc.) of an image bearing member has been devised in the development so far. However, the electrostatic fatigue of an image bearing member greatly depends on the compositions thereof and the process conditions. From a point of development of an image bearing member, developers are required to deal with each process condition. Under these circumstances, the study on the electrostatic fatigue has hardly been made in terms of compositions of an image bearing member suitable for a short wavelength light source.
Published unexamined Japanese patent application No. (hereinafter referred to as JOP) 2002-268255 describes an image bearing member including a short wavelength light absorption layer, a photosensitive layer and a surface protective layer which are provided on a substrate. The short wavelength light absorption layer contains an N type like organic pigment and absorbs writing light having a wavelength of from 400 to 450 nm. Thereby, the occurrence of moiré is prevented. Due to this method, it is certain that the reflection from a substrate or a short wavelength light absorption layer does not occur so that the occurrence of moiré can be prevented.
However, this method involves a drawback in that an organic pigment is used. The short wavelength writing light source for use in the present invention has a large energy, which is equivalent to the second absorption band level energy of some of such organic materials. The mechanism of the generation of optical carriers of an organic material is described using the scheme shown in FIG. 1.
Most of the known mechanism so far is based on the reaction formed of the two steps (a first step from light excitation to generation of geminate pairs and a second step from the generation of geminate pairs to generation of free carriers) as shown in FIG. 1. The (A) charge generating material absorbs light and is excited to a higher excited state and the geminate pairs are generated when the charge generating material is excited to a certain energy level. This certain energy level is the minimum excited singlet state (S1). Optical carriers are hardly produced by light having an energy smaller, which means light having a longer wavelength, than the difference between the ground level (S0) and the minimum excited singlet state (S1).
In contrast, in the case of the irradiation of light having an energy greater, which means light having a shorter wavelength, than the difference between the ground level (S0) and the minimum excited singlet state (S1), the energy state is excited to an energy level (S*) (an excited singlet state) than the minimum excited singlet state (S1) and rapidly relaxed to the minimum excited singlet state (S1) and the geminate pairs started to be formed. The energy corresponding to the difference between S* and S1 is thermally relaxed (represented as extra energy shown in FIG. 1).
When this extra energy is extremely large, a reaction other than the thermal relaxing may occur directly at the excited state of a charge generating layer because the thermal relaxing is not sufficient to absorb the extra energy. For example, oxidization and discomposition of the material occur. When an organic pigment is used in a short wavelength light absorption layer, the organic pigment is used therein with a high density than in a charge generating layer to prevent reflection of the writing light almost perfectly. This has a large impact. Resultingly, the technology described in JOP 2002-268255 can prevent the occurrence of moiré but cannot continuously restrain the rise of the voltage at irradiated portion.